The present invention relates to an antenna device provided for a portable radio, such as a portable phone (cellular phone), and in particular to an antenna device capable of transmitting and/or receiving data in two different frequencies.
International Publication WO95/12224 discloses an antenna device provided for a portable radio, such as a portable phone, which can be used during carriage. The antenna device resonates at two different frequencies to make it possible to use the portable radio in a plurality of frequency bands.
The antenna device 100 will be explained with reference to FIGS. 7 to 9. As shown in FIG. 7, the antenna device 100 comprises a fixed antenna part 103 including a first helical antenna element 101 and a second helical antenna element 102. Each of the first helical antenna element 101 and the second helical antenna element 102 is made up of a pair of loading coils wound spirally around a cylindrical insulated sleeve 104. The pairs of the loading coils are further wound spirally together.
The base end of each helical antenna element 101, 102 is connected with a conductive sleeve 105 mounted on the lower end of the insulated sleeve 104 so as to be connected electrically with a transmitting and receiving circuit of a portable radio (not illustrated) via the conductive sleeve 105.
FIG. 8 is an equivalent circuit diagram of the fixed antenna part 103. The first helical antenna element 101 and the second helical antenna element 102 are resonated at different resonant frequencies f.sub.11, f.sub.12 according to the connection with the conductive sleeve 105. Accordingly, as shown in FIG. 9, the fixed antenna part 103 is capable of transmitting or receiving data in a frequency band wider than that of an antenna to be resonated at a frequency f.sub.10 (shown by the chain line in the figure).
The antenna device 100 is also provided with a sliding antenna part 107 made up of a bar-like rod antenna element covered with an insulated cover 106 in the upper part. The sliding antenna part 107 is insertable into the insulated sleeve 104 and the conductive sleeve 105, and used as a whip antenna resonated at a frequency f.sub.10 when the sliding antenna part 107 is pulled out from the portable radio so as to connect the lower end thereof electrically with the conductive sleeve 105.
Therefore, the antenna device 100 is capable of transmitting or receiving data in either of a state where the sliding antenna part 107 is stored and a state where it is pulled out. As shown in FIG. 9, a radio signal can be transmitted or received in a wide band including the two frequencies f.sub.11, f.sub.12 in either state.
On the other hand, frequency bands to be used in recent mobile communication equipment differ. For example, it is a 900 MHz band for portable phones and automobile phones, and it is a 1.9 GHz band for PHS (Personal Handy-phone System). Therefore, a portable radio usable in either communication system is desired so that an antenna device compatible with dual bands, capable of transmitting or receiving data in either band is required.
The antenna device 100 achieves transmission and receipt in a wide band according to the fixed antenna part 103 by adjusting the two helical antenna elements to be resonated at the two comparatively close frequencies f.sub.11, f.sub.12. Therefore, the antenna device 100 cannot meet the above-mentioned requirement. In order to meet the requirement with the configuration the same as the antenna device 100, two helical antenna elements 101, 102 having different expanded lengths need to be connected parallel and directed outward with respect to the conductive sleeve 105, corresponding with two completely different frequency bands f.sub.1, f.sub.2 so that the appearance of the portable radio is spoiled as well as the number of parts is increased to complicate the assembly.
In order to solve the problem, the applicant has proposed an antenna device 110 to be resonated at completely different frequency bands f.sub.1, f.sub.2 by providing two helical antennas 112, 113 connected in series on a feeding metal fixture 111 to elongate linearly and adjusting the expanded length and the winding pitch of each helical antenna as shown in FIG. 10 (Japanese Patent Application No. 10-164616).
However, the antenna device 110, which is required to form helical antennas 112, 113 such that they are resonated in the two respective bands f.sub.1, f.sub.2 to be used, involves a problem in that a processing step of the loading coils is needed, and thus it is difficult to mass-produce a helical antenna with the designed value but the resonant frequency can be irregular per each antenna device 110.